Vehicle heating systems are used to heat the interior of a closed vehicle independently of external influences. In addition to the increase in comfort thus achieved, such systems also assume functions relevant to safety. Above all in this context there is clear vision through the glazed portions of the vehicle. Thus, for example, low external temperatures lead to condensation of the water vapor in the interior on the windows. As a result, these can mist up or even ice up, obscuring or even preventing vision.
Particularly in the case of motor vehicles, a not insignificant part of the energy obtained from the fuel in the internal combustion engine is converted into heat. To ensure economical and, in particular, long-lasting use of the internal combustion engine, the engine must be operated within a particular temperature range. In order to keep within this range, especially at the top, appropriate cooling measures are required. For this purpose, air cooled internal combustion engines have areas with a generally rib-type external structure. By means of the surface area enlarged in this way, some of the operational heat is released to the ambient air. In the case of water cooled internal combustion engines, in contrast, the coolant flowing around the engine block and the cylinder head via ducts initially absorbs a large proportion of the waste heat that arises. To prevent continuous heat absorption and overheating of the coolant, this coolant is then passed through a suitable cooler. In the process, some of the heat is released to the ambient air via the cooler, which is designed as a gas/coolant heat exchanger.
Ever since the introduction of water cooling, the coolant used has also been used to heat the interior of the vehicle. For this purpose a heating heat exchanger has been integrated into the high-temperature cooling circuit of the internal combustion engine in addition to the cooler. Designed as a gas/coolant heat exchanger, this enables the heat energy contained in the coolant to be released to the air in the interior of the vehicle. For this purpose, air is drawn in from the outside or from the interior and directed past the heating heat exchanger or through the latter. During this process, the air absorbs some of the heat energy before being directed into the interior of the vehicle.
To increase the effectiveness of modern internal combustion engines, they are increasingly being supplied with compressed combustion air. The turbochargers or gas-dynamic pressure wave machines used for this purpose are driven electrically or by the flow of exhaust gas from the motor vehicle. The aim is to compress the induced air in order to increase the proportion of oxygen (O2) per unit volume and thus to increase reactivity. In this way, more effective combustion of the fuel is achieved. Moreover, modern internal combustion engines can in this way develop a high power, despite smaller displacements.
Compression of the intake air is associated with an increase in the temperature thereof. To further increase enrichment with oxygen, the air compressed in this way must be cooled before being introduced into the internal combustion engine.
Further developments envisage the use of charge air coolers, which remove some of the heat energy from the combustion air. In order to release the heat withdrawn from the combustion air to the environment, the charge air cooler can also be incorporated together with another cooler into a separate low-temperature cooling circuit.
A charge air cooler system which has an integrated heating device is known from the as yet unpublished DE 10 2013 203 643.4. The turbocharger arrangement used for this purpose includes an internal combustion engine that can be pressure charged by means of at least one turbocharger and a charge air cooler which is arranged between the turbocharger and the internal combustion engine and is situated in an intake section. In this case, the charge air cooler is coupled to a low-temperature cooling circuit, while the internal combustion engine is coupled to a high-temperature cooling circuit. In order to remove the condensate which sometimes arises with pressure charged internal combustion engines, the heat energy of the high-temperature cooling circuit is used to heat the low-temperature cooling circuit.
DE 10 2013 206 082.3, which is likewise not yet published, discloses an engine system for a vehicle which includes an internal combustion engine, a turbocharger and a charge air cooler. The internal combustion engine and the charge air cooler are furthermore coupled to respective cooling circuits. In order to allow the operating temperature of the internal combustion engine to be reached in as timely a manner as possible, a number of modifications with respect to the path within the cooling circuits are proposed, wherein a low-temperature cooling circuit is used to heat a high-temperature cooling circuit.
In order to enable the internal combustion engine to be heated up as quickly as possible when started cold, the high-temperature cooling circuit thereof is initially reduced in size. In this case, most of the coolant flows around the engine block and the cylinder head without releasing the heat energy thus absorbed to a heat exchanger. After a predetermined coolant temperature has been reached, the small cooling circuit is enlarged by incorporating any heat exchangers. Especially during the cold running phase of the internal combustion engine after the cold starting of the latter, there is thus a relatively long period of time during which there is not enough warm coolant available to heat the interior.
Diesel and gasoline engines, which are becoming ever more efficient with advances in development, allow increased conversion of the fuel into the desired kinetic energy. Owing to the consequent increase in efficiency, there is a simultaneous decrease in the heat which arises in the combustion process. Consequently, the waste heat from modern combustion engines is sometimes no longer enough to ensure heating of the interior to a sufficient level. For this reason, there is a need to use auxiliary heaters, which generate the heat energy that is lacking when required. For most markets, “PTC” heating elements are typically used for this purpose, these converting electric energy into heat energy. The use thereof is due especially to the fact that the PTC heating elements manage without exposed heating wires and thus do not represent a possible safety hazard.
PTC heating elements or PTC thermistors are ceramic semiconductors, the electric resistance of which varies abruptly from time to time as a function of the temperature. Here, “PTC” stands for “Positive Temperature Coefficient”, indicating a reduction in electric resistance at low temperatures. As a result, there is a kind of self-regulation since the heat energy generated by means of the PTC heating elements depends on the already existing temperature. As soon as a particular temperature is exceeded, the electric resistance of the PTC thermistors increases in such a way that no more additional heat energy is generated.
Despite these advantages, the use of PTC heating elements in vehicles is correspondingly costly. Moreover, the vehicle weight and complexity of the vehicle heating system are increased. In addition, an auxiliary heater of this kind requires electric energy which, in turn, must be made available by the generator operated by the internal combustion engine. Since other loads on board the vehicle must also be supplied, the electric energy available for one or more such auxiliary heaters is limited, especially when the internal combustion engine is idling. In combination with modern internal combustion engines, which are more effective overall and, at the same time, produce less waste heat, the required heating systems for heating the interior therefore also still leave room for improvement.